The present invention relates to a blade fold system for a rotary-wing aircraft, and more particularly to a bearing housing mounted rotor blade bearing assembly which folds relative a rotor hub assembly to minimize pitch lock system requirements.
While the flight capabilities of rotary-wing aircraft makes them effective for a wide variety of missions, operation of rotary-wing aircraft in certain environments may be limited by the overall structural envelopes thereof. The radial dimensions of rotary-wing aircraft main rotor assemblies results in rotary-wing aircraft having relatively large structural envelopes which may impact their utility in some environments.
Rotary-wing aircraft, particularly military rotary-wing aircraft utilized for maritime flight operations, may be required to conduct operations from ships for extended periods of time. Shipboard space is generally at a premium, and the structural envelopes of rotary-wing aircraft may require a relatively significant allocation of such limited space. Furthermore, strategic and tactical considerations in the military utilization of rotary-wing aircraft has led to a requirement for rotary-wing aircraft having main rotor assemblies that may be readily reconfigured for rapid deployment, routine transport, and/or stowage through reduction in structural envelope.
Several options are available to reduce the structural envelope of rotary-wing aircraft to facilitate rapid deployment, routine transport, stowage, and reduce the vulnerability thereof to environmental conditions. One option is to design the main rotor assemblies thereof so that the main rotor blades fold relative the main rotor hub assembly. Typical folding systems include a blade spindle attached to a rotor hub assembly and a fold hinge assembly attached to the blade spindle in series (spindle-to-hub and spindle-to-hinge).
Blade fold systems may be a challenge because the blade retention/pitch bearing needs to be isolated from the moments generated by the blade when in the folded position. Various blade fold systems include a pitch-lock system.
A rotor blade at rest in a flight position experiences 1 G static droop and 1 G static mass moment. For a semi-rigid rotor, these shears and moments can be reacted by the blade retention/pitch bearing. When the rotor blade is folded, the 1 G static mass moment (which is flatwise) becomes a torsional moment about the pitch bearing degree of freedom. Without a robust pitch lock system, the rotor blade will drop to the ground. The pitch lock allows the torsional moment to be reacted by a force couple between the blade retention/pitch bearing and the pitch lock point.
For an articulated rotor, the 1 G droop moment is reacted by a droop stop. The droop stop is angled to keep a rotor blade elastomeric bearing in compression. When the blade folds, the 1 G static moment becomes a torsional moment (same as semi-rigid) which must be reacted by the pitch lock and the elastomeric blade retention/pitch bearing. Since the bearing is no longer in compression, it cannot carry significant shear load. A centering bearing or centering ring or dual pitch lock is often used to protect the spherical blade retention/pitch bearing. Such pitch lock and droop stop systems often require a substantial structure to assure proper operation.
Furthermore, some other rotor systems such as a servo-flap rotor system may further complicate pitch-lock during blade fold because there may be no direct control link to the rotor blades.
Accordingly, it is desirable to provide a compact rotor blade folding system which protects the blade retention/pitch bearings within a rotor system with a minimum of support structure.